There is known a method of monitoring the state of an extended cylindrical shell, e.g. a marine riser in offshore drilling (ARG-Telefunken, BRD. "Position Measuring System for Offshore Installations. System design and mathematical description", 1980, 10 pp.).
The known method includes the steps of selecting a sensitive element resposive to variation of the monitored parameter, e.g. the geometry of the surface of an extended shell, representative of the state of this shell, providing an extended line of transmission of wave energy carrying information on variation of the monitored parameter, matching the selected responsive element and extended line of transmission of wave energy, positioning the matched responsive element and extended line of transmission of wave energy in the zone of monitoring lengthwise of a specified running coordinate along which the monitored parameter is expected to vary, shaping and feeding to the input of the extended line of transmission of wave energy a time-modulated reference signal transformable in its propagation along this line in accordance with the variation of the monitored parameter representative of the state of the extended shell, measuring the parameters of the transformed reference signal at the output of the extended line of transmission of wave energy, and employing the measured parameters of the transformed reference signal for determining the mechanical characteristics of the state of the extended shell along the specified coordinate along which the monitored parameter varies. These basic steps of the method of AEG-Telefunken are specifically implemented, as follows.
In this known method, the monitores parameters representative of the state of an extended shell--a marine riser--are the angle of deviation of the axis of the marine riser from a vertical line and twist angles in the horizontal plane of the uppermost and lowermost parts of the riser. Hence, the selection of responsive elements is carried out by providing special-design sensors responding to variations of the above angles, i.e. inertia-type inclinometers and magnetic compasses. Then the extended line of transmission of wave energy is provided in the form of a shielded electric cable with a polyethylene protective sheath. The matching of the selected sensors and cable is effected by providing inductive coupling therebetween. The matched sensors and cable are secured on the surface of a marine riser lengthwise of its axis. To obtain sufficient data on the geometry of the longitudinal axis in the three-dimensional space, the sensors of deviation of the axis from the vertical line are arranged and secured along two orthogonal generatrices on the surface of the marine riser. Then the reference electric signal time-varying at a 400 Hz frequency is fed to the input of the cable. While propagating along the cable, the reference signal supplies the sensors with power through their inductive coupling and collects from them the information on the angles, but at a frequency substantially higher than the carrier frequency of 400 Hz. The parameters of the transformed reference electric signal are measured at the output of the cable, the information thus obtained is decoded, and the data on the angles of twist and deviation of the longitudinal axis of the riser from the vertical line are employed for determining the mechanical or physical characteristics of the state of the riser. The characteristics thus obtained define unambiguously the strained/deformed state of the marine riser and the relative positions in space of its two extreme points for dynamic positioning.
The essential limiting traits of the known method of monitoring the state of an extended cylindrical shell are, as follows.
The method is unsuitable for monitoring extended shells with high resolution and at considerable distances from the monitored object, on account of limited information capacity of the extended line of transmission of wave energy, to say nothing of the reliability of information transmission being severely affected with a growing length of the line and correspondingly growing levels of disturbances and voltage losses in this line. The employment of descretely positionable responsive elements in the form of angle or displacement sensors would not provide in principle for continuous measurement of the distribution of a monitored parameter, it only being possible to approximate the distribution of the parameter from discretely obtained measurements along a specified coordinate, e.g. of the angles of deviation of the surface of the marine riser from the vertical line. Therefore, it is impossible in principle to monitor the mechanical characteristics of a shell that are continuous both in space and in time, which inevitably results in approximation errors. Furthermore, the overall monitoring accuracy is also affected by considerable errors of the sensors themselves. The matching of the responsive elements with the extended line of transmission of wave energy in the electro-mechanical manner, i.e. through inductive coupling and a special type of securing a responsive element with respect to the line, with sensors of different kinds involved, necessitates a complicated procedure of interrogation of these sensors with the use of specifically manufactured costly electronic hardware in underwater makes which is not entirely reliable.
Moreover, the emplyed sensors of angles of deviation of a generatrix of the surface of a marine riser from the vertical line impose substantial limitations upon the dynamic measurement range, which would not allow to monitor the surface of marine risers in their considerable deformation. Neither is it possible to use one and the same method of the above-discussed type for monitoring different kinds of extended shells, or else to enhance the resolution of the monitoring and measuring operation without increasing the number of the sensors and reducing their spacing.
The closest prior art of the present invention by its technical essence is the method of monitoring the state of an elongated object which can also be an extended shell (PCT/SU 88/00082), including the steps of selecting an extended line of transmission of wave energy providing for propagation of signals therein in the form of modes with known space- and time-related patterns of physical fields, positioning the selected line of transmission of wave energy on the surface of an extended shell along a specified running coordinates S, defining in the extended line of transmission of wave energy at least one reference channel and at least one measurement channel with known values of deceleration of phase velocities of the modes in each of these channels, providing for directional interaction lengthwise of the extended line of transmission of wave energy of the fields of the modes of the at least one reference channel and at least one measurement channel in accordance with varying geometry of the surface of the extended shell, for producing in the measurement channel a signal varying in the course of propagation of a signal in the reference channel in accordance with the varying geometry of the surface of the extended shell, representative of the state of this shell, shaping a time-modulated reference signal in the form of oscillations of physical fields and transforming these oscillations into a signal with predetermined space- and time-related patterns of the fields of the modes, converting the fields of the modes at the respective outputs of the at least one reference channel and at least one measurement channel of the extended line of transmission of wave energy into exclusively time-dependent electric signals, extracting the amplitude of the electric signal at the output of the reference channel, amplifying the electric signal at the output of the measurement channel in inverse proportion to the value of the amplitude of the electric signal at the output of the reference channel, employing a linear scale transform relating the value of the difference between the decelerations of the phase velocities, respectively, of the modes of the reference and measurement channels of the extended line of transmission of wave energy to the running time of monitoring and to measurements of the values of the specified running coordinate lengthwise of the extended line of transmission of wave energy, and determining the geometry of the surface of the extended shell.
The last-described known method of monitoring the state of an extended shell, which is the closest prior art of the present invention, has, however, several inherent limitations.
In this method of the prior art, as an extended line of transmission of wave energy is positioned on the surface of an extended shell, e.g. a curving pipeline, there would not be enhanced the accuracy of monitoring the geometry of the surface of the extended shell on account of the absence of element-wise detection of integral characteristics of the surface of the extended shell, with subsequent composition of these characteristics with the aim of unambiguous determination of the geometry of the surface of the extended shell, and, consequently, of its strained and/or deformed state.